When a wind turbine facility or the like is mounted on an offshore structure in a water area having a deep water depth, a semi-submersible type, a tension leg platform (TLP), or the like may possibly be employed. In such cases, the wind turbine facility or the like and the offshore structure are integrally assembled and subjected to test operation onshore, and then towed to an installation site, and moored by a mooring system.
In the case of the spar-type offshore structure, the spar is an offshore structure floating in an upright standing state like a fishing float, and a large part of the float is submerged under the sea level by injection of ballast water or the like. The spar is relatively unlikely to swing, and can support a heavy upper structure. In the case of a spar-type offshore structure equipped with a wind turbine facility, the draft of the float may be as deep as approximately 40 m to 80 m, and the float portion becomes a large structure of approximately 1000 t (ton).
As the spar-type offshore structure, a float-type fluid force utilizing system is provided as described in International Publication No. 2013/065826, for example. In this system, a horizontal-axis wind wheel or a vertical-axis wind wheel for utilizing wind force is disposed at an on-water portion thereof, and a horizontal axis water wheel or a vertical axis water wheel for utilizing tidal current force is disposed under the water, and which utilizes the horizontal axis water wheel or the vertical axis water wheel as ballast.
In addition, regarding the spar-type offshore structure used as the offshore wind power generation facility, not only one but a large number of spar-type offshore structures are moored in an installation site suitable for offshore wind power generation. For example, as described in Japanese patent application No. 2004-176626, an offshore wind power generation facility is proposed, in which a plurality of floats each supporting a wind turbine facility in an upright standing state are coupled to one another by a mooring chain provided with an intermediate sinker in a middle, and an appropriate float is further connected with a mooring chain provided with a mooring anchor at one end and with an intermediate sinker in a middle.
In the offshore wind turbine facility, as an example of the arrangement of the plurality of offshore power generation facilities and mooring bases, there is shown an arrangement in which floats, which are offshore power generation facilities, are arranged at vertices of a regular hexagon in a plan view, and the floats are moored by a central offshore structure, which is arranged at the center of the regular hexagon in the plan view, and mooring anchors, each of which constitutes a regular triangle with two adjacent floats in the plan view, by using mooring chains in seven directions, which are provided with intermediate sinkers and connect the one central float and the respective six floats, the mooring chain in one direction is locked with the central float and the mooring chains in the remaining directions are locked with the six mooring anchors.
On the other hand, generally, as illustrated in FIG. 1, one offshore structure is often moored by mooring lines in three directions or more. The mooring is often formed of a drag anchor, as well. In such mooring, the mooring line in one direction formed of a plurality of, for example two or three mooring lines. The strength of each mooring line is set such that even when one of the mooring lines is broken under severe weather or oceanographic condition, the remaining mooring lines are not broken like a chain reaction can keep mooring the offshore structure at the original position.
For this reason, as the number of the mooring lines increase, a breaking strength required for each mooring line and a holding power required for the drag anchor increases, bringing about a problem of high costs for mooring the offshore structures.